According do Google's Guide: http://stereofox.com/post.php#!idPost=5326 would get crawled at http://stereofox.com/post.php?_escaped_fragment_=idPost=5326. Your server would have to respond to that URL the same way that it used to respond to http://stereofox.com/post.php?idPost=5326.

You should then also 301 redirect http://stereofox.com/post.php?idPost=5326 to http://stereofox.com/post.php#!idPost=5326